1. Field of the Invention
This invention relates to a mobile telephone system and a method of providing a communication channel for mobile telephony, and has particular but not exclusive application to digital mobile systems, for example Global System for Mobile Communications (GSM) systems.
2. Related Art
In a conventional GSM mobile cellular telephone system, a mobile station (MS) such as a mobile telephone handset communicates through a radio link with one of a number of fixed base transceiver stations (BTS) distributed at different geographical locations throughout the area of coverage of the system. Clusters of BTS's are connected to a base station controller (BSC) through landlines. Subgroups of the BTS's may be coupled through a hub to the BSC. Groups of BSC's are coupled by means of respective landlines to a mobile switching centre (MSC). The MSC, in addition to being connected to other BSCs, has a connection to a public switched telephone network (PSTN). The system permits communication to be established between two MSs, or between a MS and a conventional telephone connected to the mobile system through the PSTN, or to another network.
Conventionally, the landlines that are used to connect each BTS to its hub, or direct to the BSC, are leased from a terrestrial telecommunications system provider. The leased line needs to have sufficient bandwidth for the peak data traffic flows that occur in use in the network, with the result that during periods of low system utilisation, the bandwidth of the lines is not fully utilised. However, the leased line is rented on a continuous basis rather than on its level of utilisation and so the cost-effectiveness of the lines decreases during periods of low use. A typical so-called E1 (or T1 in the USA) leased digital line offers a bandwidth of 2 MB/s whereas typical data rates between a BTS and its associated hub are of the order of 1 MB/s or less.
Terrestrial system providers also offer digital leased lines, which can be rented with a fixed bandwidth of N.times.64 kb/s, where N is an integer typically from 1-30. These lower-bandwidth leased lines are suitable for providing the connection between each BTS and the BSC, either via a hub or directly. The leased line connection to a BTS suffers from a disadvantage that in order to provide sufficient bandwidth when the BTS is operating close to its maximum utilisation level, the bandwidth needs to be selected to be higher than that needed for the traffic levels that occur for most of the time. This increases the cost of the leased line. Similarly, an E1/T1 leased line which can deal with peak levels between the hub and the BSC may provide excessive capacity for average traffic levels that can occur during normal levels of utilisation of the system. Similarly, if the capacity of the BTS is expanded, additional bandwidth may be needed, i.e. another leased line may be required which will be inefficient and costly for average traffic flow, but needed in order to handle peak traffic flow rates.
It is known, for example from European Patent Application No 0398183 (Northern Telecom) for a private telephone system having a communication link (e.g. a private link) for providing a first communication path for signals communicated between two nodes of the system to provide the nodes with further connections for communication with a communications network (e.g. a public network); and control means operable to selectively provide a second communication path, through the communications network, for augmenting the bandwidth available on the first communication path for signals communicated between the nodes. For each call, traffic may be carried on either the first path or the second path, but control signalling between the mobile station and the network termination is always carried on the first path. This principle can be applied to the connections between a base transceiver station and control station of a mobile telephone system, by providing the communication link as a leased line, so as to provide the first communication path, and providing the communication network as an integrated services digital network (ISDN) so that the second communication path can be selectively set up through the ISDN to provide additional bandwidth for the signals in the event of high system utilisation. The ISDN is billed to the user according to the amount of time it is used and so the bandwidth available for the signals can be augmented by providing the second communication path, with the cost only being incurred for the time it is used for the second path. In this way, the leased line can be selected to have a bandwidth which can be used at a relatively high utilisation rate during normal operation, because in the event of higher utilisation rates, the second path can be selectively connected to handle the additional `peak` bandwidth requirement that occurs during high levels of system use.
However, although the links between the base transceiver stations and control stations form a private network, they carry tariff-bearing mobile telephone calls. This causes a problem, because when a call is connected through the system, the time required to establish connection may typically be greater through the first path than through the second path, for example where the first path comprises a leased line and the second path consists of an ISDN. Billing to the user is established by a "call-connect" signal which is transmitted back, typically to the BTS in order to signal that the connection has been made and that billing can commence. Discrepancies could arise, either when the call-connect signal over the leased line arrives first, in which case the MS user would be charged for the time between the arrival of the call-connect signal and the actual time of connection over the ISDN. Conversely, if the call connect signal over the ISDN path were to arrive first (or the call-connect signal over the leased line fails to arrive at all, for example because the call is not answered) the mobile network operator will be charged for the ISDN link before the MS user is connected, thereby incurring an unnecessary cost to the mobile network operator. Note that the ISDN call-connect signal only relates to the connection between the BTS and the BSC.